The present invention relates to a single chip processor, and in particular it relates to a single chip processor having a variable instruction cycle.
There have been many single chip processors which have a memory area storing a program inside a chip of a single processor while being designed to enable the expansion of a program area to the outside of the chip so as to prepare for deficiency of the capacity of an internal memory This consturction is adopted because a memory capacity can not be incorporated in the chip sufficiently for expected applications due to such factors as chip size, consumed power, etc. and it is unavoidable to rely on an external memory in order to secure a capacity sufficient for said expected applications.
In almost all of such single chip processors having a program memory also outside the chip, the same access time is taken in the case when a memory incorporated in the chip is employed as in the case when an external memory is employed.
In a Digital Signal Processor TMS32020 manufactured by Texas Instrument Corporation, for instance, access is made at a slower speed by using a control signal from a pin when an external program memory is employed than when an incorporated memory is employed Even in this case, however, the access time of the incorporated memory is in accord with the access time of the highest speed of an external memory.
As stated above, the time for access to the incorporated memory is the same with the time for access to the external memory in the conventional single chip processors having external memory. In these conventional single chip processors, consequently, an instruction cycle is limited by the operating speed of the external memory, and therefore an improvement in the performance in regard to processing speed can not be attained. This is because the same instruction cycle is given to operations for which the incorporated memory is employed, although this memory is capable of operating the chip at a much higher speed than in the case when the chip is operated according to an instruction inputted from the external memory.